Centrifugal compressor



May 3, 1949. s. A. SHULER', JR

CENTRIFUGAL COMPRESSOR 2 Sheets-Sheet 1 Filed March l9, 1946 Inventor: 0rd. A.Shuler-,dt;

H is Attorney.

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y 1949. s. A. SHULER, JR

CENTRIFUGAL COPRESSOR 2 sheets-sne't 2 Filed March 19. 1946 49 Inventor: 3 .zr5anfor-d A.5hu.|r;dr;

7? by His Attorney Patented May 3, 1949 CENTRIFUGAL COMPRESSOR Sanford A. Shuler, Jr., Schenectady, N. Y., alsignor to General Electric Company, a corporation of New York Application March 19, 1946, Serial No. 655,517

My invention relates to a centrifugal turbomachine for pumping a fluid, particularly to a two-stage centrifugal compressor for compressible gases. The embodiment described herein is specificially intended for supercharging the .cabin of a high altitude aircraft, although it may be used for supplying fluids under pressure for other purposes.

An object of my invention is to provide an improved two-stage centrifugal pump which is compact, light in weight, yet very sturdy, and easy to manufacture.

Another object is to provide an improved twostage centrifugal compressor particularly adapted for supercharging aircraft cabins with a comparatively small volume flow of air at comparatively high pressure ratios.

Another object is to provide a multi-stage centrifugal pump arrangement having an interstage passage of good aerodynamic efliciency.

A further object is to provide an improved twostage centrifugal pump rotor having two impellers arranged back-to-back with special sealing means for preventing leakage of fluid from the higher pressure stage to the lower. Still another object is to provide an improved diffuser arrangement for a two-stage centrifugal compressor or pump having impellers of the type described.

' Other objects and advantages will be apparent from the following description taken in connection with the accompanying drawings, in which Fig. 1 is an exploded perspective view of the components, partly in section, which make up my improved compressor; Fig. 2 is an end view of the assembled compressor; Fig. 3 is a sectional view taken on the plane 3--3 in Fig. 2; Fig. 4 is an enlarged detail view of a modification of the exit portion of the impellers and the entrance portion of the diffusers; and Fig. 5 is a still further modification of the impeller exit arrangement.

Referring now to Fig.' 1, my compressor comprises a rotor l housed in a casing consisting of four component parts, a first casing 2 which serves to support one, end bearing and defines the first stage inlet passage, a casing 3 which forms the first stage discharge scroll and a first portion of the interstage passage, a diffuser assembly l, and a casing 5 which forms the other 'end bearing support and defines the remainder of the interstage passage and the second stage discharge scroll.

- By reference to Figs. 1 and 3, it will be seen that the rotor comprises a shaft assembly 6 having mounted thereon a single rotor, shown at l in 3 Claims. (01. 230 -130) Fig-. l and comprising a hub with a radially extending web portion I, a first stage shroud 8 and a second stage shroud 9 on opposite sides of the common web I. Secured to members 1 and 8 are a plurality of circumferentially spaced radially extending blades Ill which define the first stage impeller fiow paths. The second stage flow paths are formed by radially arranged blades ll secured to web I and shroud 9. While impeller l bears a superficial resemblance to what is known in the art as a double inlet impeller, it is actually two independent single inlet impellers, of materially different aerodynamic design, arranged coaxially and in back-to-back relation. It should be particularly noted that the first stage or low pressure impeller has an overall or tip diameter materially less than that of the second stage or high pressure impeller.

While the impeller l is disclosed in the drawings as an integral cast impeller, it will be readily appreciated by those skilled in the art that impellers of other types made by other methods of fabrication may be used. For instance, the

impeller may be of the shroudless or open type, in which the hub and web portion and the respective blades l0 and H are machined from a forged disk, and portions of the stationary casing form close clearances with the edges of the blades so as to prevent leakage from the impeller flow paths. Such an open type impeller is disclosed, for instance, in U. S. Patent to Kenton D. McMahan, No, 2,380,772, issued July 31, 1945. It will be obvious that separate impellers of such a type could be mounted on a common shaft in back-to-back relation, in the manner shown in Fig. 3.

The arrangement of the inlet casing 2 may be seen by a comparison of Figs. 1, 2 and 3. Air from an inlet duct (not shown) enters the flanged inlet opening l2, into the annular inlet passage 13 and to the inlet of the first stage impeller, as indicated by the arrows It in Figs. 1, 2 and 3.

As can best be seen in Fig. 3, casing 2 also serves to support a first end bearing l5, which may be of any suitable type. A shaft seal assembly I6 is provided adjacent the bearing IE to prevent lubricant entering. the fluid flowing through the compressor. Further sealing effect may be provided by grooves in the casing cooperating with a portion of shaft't to form a labyrinth seal l1.

The periphery of the first stage impeller forms a close clearance with the inlet edges of a diffuser, formed by a radially extending plate l8 lying in a plane normal to the axis of the compressor and having formed integral therewith an axially spaced parallel annular plate IS, with a plurality of circumferentially spaced radially and tangentially extending vanes 2t) connecting side plate l9 to the backplate I 8. Vanes 20 with the sideplates I 8, l9 form a vane type diffuser, which is well known in the art, being disclosed for instance in U. S. Patent to S. A. Moss, No. 2,157,002, issued May 2, 1939.

Casing 3, which forms the first stage discharge scroll 3a, is provided with an inwardly extend! .ing 'annularportion 2| which engages theouter resent the exit end of the first stage discharge scroll and the beginning of the interstage passage. The section A is seen in elevation in Fig. 3 and in edge view in Fig. 2. As may best be seen in Fig. 1, casing 3 also forms a portion of the interstage passage extending from the cross-section A and making arather sharp 90 degree turn, ending at the section B.v Crosssection B may be seen in perspective in Fig. 1, in elevation in Fig.2, and in edge view in Fig. 3.

Fluid flows through the first stage scroll 3a in the manner indicated by arrows 30.

The elongated rectangular discharge opening 23 (at section B in Fig. 1), which forms the exit of the fluid flow path defined by casing 3, cooperates with a similar opening 24 in the diffuser backplate l8.

Casing forms the remainder of the interstage passage, beginning at the opening 24 in diffuser backplate I 8, making a sharp 90 degree turn at 25, and having walls defining a'radially inwardly extending conduit 26 which communicates with the annular second stage inlet chamber 21. Fluid discharged from the opening 23 in casing 3 flows in the direction of arrows 23, 23.

(Fig. 2 and Fig. 3) to the inlet of the second stage impeller.

After flowing through the radial passages defined by impeller blades li, the fluid is received in a second vane typediifuser formed by the common backplate l8 and a second parallel annular side plate 30, with vanes 3| (Fig. 1) therebetween. The diffuser backplate l8 forms a "backbone. lending rigidity to the whole casing assembly, while at the same time helping to define both the first and second stage diffuser passages.

The second stage discharge scroll 32 formed by casing 5 begins with a comparatively .small cross sectional area, as may be seen. in the upper portion of Fig. 3, and progressively increases [in area around the circumference of the second stage impeller, toa maximum at the flanged discharge opening 33, as indicated in Fig. 2. Fluid discharged from the second stage difiuser flows through the scroll "32 as indicated by the arrows 34 (Figs. 2 and 3).

4 common circumferential row of bolts through the flange 22 of casing 3 and similar flanges 22a and 22b on casings 4 and 3 respectively. Inlet casing 2 is secured to casing 3 by means of a separate circumferential row of bolts 33 through holes provided in a bolting flange 36. The configuration of casings 2 and 3 is carefully selected so that casing 2 may be assembled to'casing 3 with the compressor inlet flange l2 in any desired angular orientation relative to casing 3.

This arrangement permits the user of the compressor to readily adapt it for any'desired angular relation between the compressor inlet duct (connected to flange l2) and the discharge duct (connected to flange 33). This is an important feature in aircraft installations of cabin supefi chargers, where there is much complex ductwork to be accommodated in a given small volume and optimum use must be made of. the space available. It should be noted that this casing arrangement of my compressor ermits a minimum overall axial length, the compressor inlet duct and the discharge duct being arranged in parallel, closely adjacent planes.

' As may be seen in Fig. 3, end casing 3 provides a support for the left-hand shaft end bearing 31 and the shaft seal assembly 38. Casing 3 may also form a labyrinth shaft seal 39. The projecting end of shaft 6 is provided with a gear 40 adapted to be driven b any suitable means,

as for instance by a gear train-from the crankshaft of a reciprocating internal combustion en; gine (not shown). Casing 5 is provided with a mounting flange 4|, by which the compressor assembly may be bolted to a suitable support, for instance the accessory drive casing of the engine from which the compressor receives its power. The Opposite end of the compressor assembly may be providedwith a circumferential flange 42, to which may be secured other accessory devices, such' as an electrical generator (not shown) which may have its shaft coupled to the compressor shaft by any suitable means at the fitting Attention is particularly directed to the arrangement and shape of the interstage passages in' my compressor. It is well known among those skilled in the centrifugal compressor art that extremely serious energy losses can occur in the passage between. the stages of a multi-stage machine unless the greatest care is taken in the design of such passages. Because of the inherent configuration of centrifugal impellers, it has been found extremely difilcult to get the fluid discharged from the first stage impeller to the inlet of the second stage impeller without introducing appreciable aerodynamic losses. This is particularly true when space is at a premium and an effort is made to design a multi-stage compressor of the smallest possible overall dimensions. My invention solves this troublesome problem by an arrangement which has been- It will be seen that all four casings, 2, 3, 4, and v 5 cooperate to define the chamber containing the impeller I. As shown in Figs. 2 and 3. com

ponents 3. 4 and 3 are secured together by a found to be convenient to fabricate and assemble, yet extremely eflclent aerodynamically.

As indicated above, the "interstage passage begins at the section A and ends at the inlet to the second stage impeller. As has also been noted above, this interstage passage has a degree bend between sections A and B, and a assembly, it is necessary to make the interstage passage of elongated or flattened cross-section at the place where it bridges the diffuser assembly, in order to keep the outside diameter of the complete un t to a minimum. By reference to Fig. 2, it will be seen that the interstage passage crosses over the second stage discharge scroll at a location where the latter is comparatively small in cross-section.

In order to obtain an aerodynamically efficient interstage passage, the cross-section area is progressively reduced from section A around the 90 degree bend to section B, and still further reduced around the second bend from section B to section C. Concurrently with the reduction in area from section A to section-,B, it can be seen in Fig. 1 that the shape of the cross-section major axis in a direction substantially normal to the plane of the bend. This transition has an important aerodynamic effect in making the rapidly flowing fluid completely flll the passage with no boundary layer separation of the fluid from the walls of the passage. Such separation, if it occurs, results in energy-consuming eddies adjacent the walls.

It will be appreciated by those skilled in the art of fluid mechanics that the rapidly flowing fluid will readily follow the outer surface 44, and the upper surface 45 of the passage in casing 3 (Fig. l) but unless special precautions are taken, the fluid will tend to separate from the inner surface 46 and the lower surface 41. The danger of boundary layer separation is of course particularly acute when it is attempted to make the fluid turn a sharp corner in a small space. I have found that energy losses due to such boundary layer separation in the interstage passage of a centrifugal compressor can be reduced to a minresult from the fact that the change in crosssection shape produces a velocity component transverse to the direction of the mean flow path. This transverse velocity component is in the. direction of the lengthening dimension of the cross-section area, and has the effect of causing the fluid to tend to hug the downwardly curving lower surface 41, as indicated by arrow 48 in Fig. 1, and to similarly follow the inner side wall surface 49.

It is not possible to state conveniently, either verbally or. mathematically, the precise interstage passage configuration required to give the best aerodynamic efiiciency. However, having given the fundamental concept that it is necesis squeezed down to produce the elongated cross-section 23 having its longer dimension or,

sary to both reduce the cross-section area as it is necessary to provide sealing means for preventing fluid from leaking from the discharge of the second stage impeller back into the inlet of the flrst stage diffuser, which is of course at a lower static pressure. The means by which I prevent this leakage is as follows. As noted above, the first stage impeller has an appreciably smaller tip diameter than the second stage impeller. As may be seen in Fig. 3, the impeller web portion 1 is provided with a circumferential beveled surface 49, shown to an enlarged scale in Figs. 4 and 5 and extending axially and radially outward from the discharge annulus 5| oi the -flrst stage impellerto the discharge annulus 50 of the second stage impeller. In Figs. 3, 4 and 5 the common diffuser backplate I8 is represented as havinga cooperating annular beveled surface 52 In Fig. 3, the cooperating surfaces 49 and 52 define a clearance space which decreases in cross-section area from the discharge annulus of the first stage impeller to the second stage impeller tip. With this arrangement, fluid discharged through annulus 5l is caused to be rammed into the tapered annular clearance space, so that fluid tends to flow from 5| to 50, thus counteracting the tendency of the higher static pressure produced by the second stage impeller at 50 to produce flow backwards to the first stage impeller tip 5|.

If the length of the leakage flow path defined between beveled surfaces 49 and 52 is suflicient, then this clearance space may have an increasing cross-section area, carefully designed to form a diffusing passage, (see Fig. 3) so that some of the velocity energy of the fluid entering the clearance space from impeller discharge annulus 5| may be converted into static pressure, which will have a tendency to balance the static pressure in the clearance space with that at the second stage impeller tip 50. It may also be noted that with the arrangement of Fig. 3 the fluid issuing at high velocity from the second stage impeller discharge annulus 50 may produce an ejector action tending to entrain fluid molecules in the clearance space between surfaces 49 and 52. This ejector action combines with whatever diffusing effect may be produced in the clearance space to resist the tendency for fluid to leak backwards from 50 to 5|. The abovedescribed tendency for fluid to be "rammed" into the annular clearance space, and the ejector action, can be increased by slightly offsetting the impeller axially relative to the diffuser inlet. This is shown in Fig. 4, in which the web I is oflset slightly to the left, relative to the diffuser plate I 8.

Fig. 5 shows a further modification of my impeller tip sealing arrangement in which the circumference of the web I is provided with a plurality of axially and radially extending vanes 53. These vanes serve as small auxiliary impeller blades tending to induce flow from impeller tip 5| toward the second stage impeller ti 50. This action resists the tendency of fluid to flow in the opposite direction through the clearance space between 49 and 52.

It will be seen that my invention provides an extremely compact two-stage centrifugal compressor having an improved impeller and diffuser arrangement and a novel casing structure which is aerodynamically efficient, mechanically simple to fabricate and assemble and versatile in adaptation to complex installations where space is at centrifugal compressor, it will be apparent to those skilled in the art that some or all features of the invention may be equally applicable to pumps for increasing the pressure of liquids.

What I claim as new and desire to secure by Letters Patent of the United States is:

1.- In a. multi-stage centrifugal pump having a single inlet low pressure impeller and a single inlet high pressure impeller mounted coaxially in adjacent back-to-back relation, walls defining a first discharge scroll surrounding the low pressure impeller and having an exit directed tangentially and with a cross-section of a width substantially equal to the height thereof, walls defining an annular inlet chamber for the high pressure impeller, and walls defining an interstage passage connecting the exit of the first stage scroll to said second stage inlet chamber, said passage having adjacent the first stage exit a portion forming a first bend of substantially 90 degrees to turn the fluid discharged from said exit to a direction substantially parallel with the axis of the compressor and a second portion defining a second bend of substantially 90 degrees to conduct the fluid to said inlet chamber, said interstage passage having a cross section which gradually decreases in area and simultaneously transitions to a progressively more elongated shape with a major axis extending in a direction substantially normal to the planesof the respective bends, whereby the rapidly flowing fluid is caused to fill the passage without separation from the inside walls of the bends.

2. In a multi-stage centrifugal pump having two single inlet impellers mounted coaxially in adjacent back-to-back relation, a casing with walls defining a first stage discharge scroll surrounding one impeller and having an exit directed tangentially, said scroll exit having a cross-section of a width substantially equal to the height thereof, and casing walls forming an interstage passage connecting the first stage exit to the inlet of the other impeller, said passage including a portion adjacent the first stage exit forming a bend of substantially 90 degrees to turn fluid discharged from said scroll into a direction area produces an increase in velocity in the direction of the fluid fiow and the change in cross section shape introduces a velocity component transverse to the direction'of fiow so as to cause the fluid to fill the passage without separating from the inside surface of the walls thereof.

3. In a high-emciency conduit system for conducting compressible fluids at high velocities,

walls defining a conduit portion with a substan-- tially 90 bend and a substantial concurrent deflection in a direction normal to the plane of the bend, the entrance to said bend having a cross section of a width substantially equal to its height,

the cross-section area of the passage progressively decreasing from the entrance throughout the bend and simultaneously transitioning to an elongated shape having a major axis extending in a direction substantially normal to the plane of the bend, whereby the change in cross-section area produces an increase in velocity in the directionof the fluid flow while the change in cross-section shape introduces a velocity component transverse to the direction of flow so as to cause the fiuid to ill] the passagewithout the occurrence of boundary layer separation along the inside surface of the ccnvexly curved walls.

SANFORD A. SHU'LER, JR.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES I PATENTS Number Name Date 2,135,939 Harper Nov. 8, 1938 FOREIGN PATENTS Number Country Date 541,891 Great Britain Dec. 16, 1941 

